(1) Field of the Invention
The invention relates to the manufacture of highly dense integrated circuits and more particularly to the formation of a field oxide implant within the integrated circuit.
(2) Description of the Prior Art
In the manufacture of highly dense integrated circuits, individual device structures are typically separated and electrically isolated by means of a field oxide isolation region. Prior to formation of the field oxide, field implants are formed on the substrate by ion implantation. The field oxide and field implant form part of a parasitic transistor, which should have as high a threshold voltage as possible in order to isolate the active regions.
As the size of semiconductor devices decrease, however, there have been increasing problems. In semiconductor devices, particularly MOS field effect transistor devices, there has become a problem in controlling transistor characteristics as the feature size of these devices go below one micrometer. One of the problems has been overlap between field implant and source/drain implants, or channel stop implant encroachment. This overlap is caused by the process of manufacture which includes opening the desired pattern of oxidation mask to the regions in which field oxide isolation is to be formed, ion implantation of the field implant therethrough and then oxidation to form the field oxide isolation. The field implant will deeply diffuse into the semiconductor substrate during the oxidation of the silicon surface for field isolation. During this oxidation, the dopant will encroach into the MOS field effect transistor channel regions. This encroachment will then raise the threshold voltage of the narrow channel device, making the device less useful and control more difficult, while at the same time reducing the source/drain junction breakdown voltage.
Workers in the field have understood these problems and have attempted to overcome them.
JA Application 57-39551 (Mar. 4, 1982) describes the use of a resist film smaller than the width of the hole of nitrided Si film and polycrystalline silicon film formed in pattern on a silicon oxide film on a substrate. Ions are injected through the opening and the selective oxidation is done. This is said to prevent the narrow channel effect. As semiconductor device dimensions decrease, however, dimensional control of the resist film becomes increasingly difficult.
Mitchell et al U.S. Pat. No. 4,829,019 also describes a method for overcoming channel stop encroachment. Their method includes the use of several process steps to create sidewall structures on the patterned sidewalls of the oxidation mask before the channel stop ion implantation. They also remove the sidewall structures before field oxide isolation.
It is therefore an object of this invention to provide a process for fabricating field oxide isolation pattern with field implants associated therewith that can be used for increasingly smaller dimensional elements, for example in feature sizes of 0.8 micrometers or less, and simpler processing than the prior art.